1. Field of the Invention
The present invention relates to a method and means to obtain a conference connection in telecommunication network, and in particular it relates to a conference connection well adapted for encryption/decryption facilities.
Still more specifically the invention concerns a method to transmit and receive information via a telecommunication network, between a selected plurality of mutually interconnected nodes in a network built up so that each and every node among a freely selected group of conference participating nodes, may send information to all the remaining participating nodes via an established common connection.
2. Description of the Prior Art
It is earlier well known to use conference connections in telecommunications. It may e.g. be referred to a general article by John Ellis and Bruce Townsend, published in Telesis No. 1 1987, pages 22-31: "State of the art in teleconferencing". Here both the technical solutions most used and the relevant problems are thoroughly described.
In another paper presented by D. G. Steer et al, in the conference CRYPT088, on Jul. 29, 1988 under the title: "A Secure Audio Teleconference System", a specific solution of an encrypted teleconference is described.
It should also be referred to a commonly assigned copending patent application of K. Presttun, filed on the same day as the present application and entitled, "Apparatus and Method for Establishing Conferences on a Telecommunications Network". From this application it is known to connect all the nodes which are participating in a conference, in a single ring connection.
All the above mentioned solutions comprise specific disadvantages as mentioned below.
In the Ellis/Townsend reference the main problem areas in conventional conference bridges; noise, loss and echo; are pointed out, and are more or less overcome by rather conventional methods. We shall mention that with all the solutions mentioned here we have to use a centrally arranged conference bridge, and all such bridge solutions are not ideal when encrypted speech signals are considered, as the signals exist as clear text between the participant's nodes and the central equipment and thus represent a great risk of unauthorized tapping.
In the CRYPT088 reference, there is shown a solution where the terminals which take part in the conference, are connected in series; or rather in a chain; and then a more secure crypto point-to-point connection is possible, as the decryption/encryption process may be undertaken in each terminal and not in the exchange itself.
A distributed conference system is also briefly mentioned in this CRYPT088 reference, but it is not further discussed, as it is deemed to be quite impractical.
In the ring system in the co-pending Presttun application, it is required that all echoes from the own transmitted signal is cancelled (or at least suppressed). If the conference includes a satellite connection, a large delay time is introduced, and this must be handled by a filter which represents a rather expensive solution. As the signal circulates through the ring it has to be coded and decoded many times. Each coding/decoding process introduces noise in the system, and to ensure that the total noise shall not be excessive and lead to "singing", the total ring amplification has to kept below unit.
Because of the continuous circle connection some problems arise. We remember that the signal on the circular connection represents the sum of all information received from all of the nodes. Therefore the sum signal which is received at a node has to be decoded, attenuated (so that added noise shall not accumulate), and then it goes to the receiver whereupon the local generated signal is added to the decoded signal. Then the new sum signal, comprising the local generated signal, has to be coded anew before it is retransmitted to the line.
And to avoid acoustic feed back, the echo of own generated signal has to be depressed when it returns to own terminal.
From the above it may be understood that accumulation of noise from the multiple encoding/decoding processes and cancellation of that part of the sum signal which originates from own node represent problems which must be solved in such a ring conference connection.